Keep in mind, a Space Marine, and most Eldar quad leaders, will easily have better then 5+ armour, so the AP isn't much on a problem. At most it can dissuade Ork player from charging, but even then they're likely to die from Str5 AP5 shooting. Weight of attacks can be an argument, but the quality doesn't sustain it.

On the Carbine Buckler, is the +1 attack too much? Would perhaps re-rolling failed armour saves of 1 be a better solution?

The AP can be improved with the power sword option though, which is why it's a concern. That does mean you can't use the buckler, but you then have 2 combat weapons anyway so you don't lose any attacks as a result of doing so.

The buckler change makes sense, it's a defensive tool, it makes more sense for it to provide a defensive boost. If you changed that and reduced the Shimmerblade to 1 attack instead of 2 it'd be about right. Hammer of Wrath etc only provide 1 attack so it brings it slightly in line with that too.

The AP can be improved with the power sword option though, which is why it's a concern.

I wouldn't make the shimmerblade an AP3 weapon. That would be too powerful, and would be spammed all over. Keeping it to AP5 means it works best against Orks, and other units with minimal armour. A Tau should not be able to kill a Space Marine so easily, and certainly shouldn't be able to outpace an Eldar (Hence the I7 cap)

The Battlesuits get Power Weapons as it makes sense. Its going to be fighting the tougher foes, and needs that extra edge.